The bonding of silicon wafers has been used in SOI (silicon-on-insulator) and various sensor applications. In SOI applications, the wafer bonding step is usually performed prior to any circuit fabrication. The temperature used in the SOI bonding process can be as high as 1200.degree. C. However, in the case of sensor applications there may be a temperature constraint because the bonding has to be performed after fabricating the circuitry. In a known accelerometer fabrication process, a wafer with patterned cavity on it is bonded to the device wafer to restrict the displacement range of the proof mass. The 1200.degree. C. step is too high for the device wafer which already contains circuitry metallized with aluminum. The Al/Si eutectic temperature is 577.degree. C. which requires the wafer bonding temperature to be lower than 577.degree. C.
With this temperature constraint in mind, glass has been used as the backing substrate for micromachined silicon pressure sensors because the bonding process can be performed at temperatures below 500.degree. C. Glass to silicon bonding also offers a high bond strength for the finished devices. The major drawback in using a glass-to-silicon bond is the mismatch of the thermal expansion coefficients of these two materials. Since the glass was bonded to silicon at elevated temperatures, usually higher than 300.degree. C., a built-in stress already existed in the silicon when the bonded pair was cooled to room temperature. It is known that the physical properties of silicon, such as resistivity and stiffness, depend on its stress condition. Therefore, the mismatch in thermal expansion coefficients can introduce preloaded stress problems in the bonded silicon wafer where the resistivity and/or mechanical stiffness of the silicon are the parameters to be monitored in sensor applications.
Under the same temperature constraints, Field et al, "Fusing Silicon Wafers with Low Melting Temperature Glass," Sensors and Actuators, A21-A23, p.935, 1990, deposited boron glass as the bonding agent between two wafers and was able to bond wafers together at 450.degree. C. No data on bond strength was given. However, it seems the bonded pair can pass the scribe-and-break test. Because boron glass absorbs moisture very fast and the moisture content in the glass affects the bonding results significantly, we abandoned this approach after several attempts. At even lower temperatures, others have developed procedures and self-contained equipment to bond two wafers at room temperature. However, the bond strength between the wafers was not very high.
Thus, heretofore there has been a need for a low temperature, below 500.degree. C., silicon wafer-to-wafer bonding process with enhanced bond strength.